TREE FAILURE WITH EVIDENCE OF NATURAL BRACING

I have been promoting an understanding of natural bracing in trees since I found its association with bark-included junctions in March 2016. This has led to me giving many talks in the UK and overseas, making over 1,300 arborists and arboriculturists aware of my findings. More to come, as I have quite a few talks programmed in for 2018... I have done this, as I am very conscious of the importance of this finding, for it progresses on arboricultural practice, helps to save trees and prevent failures.

Of course, there are some skeptical views out there (that I am well aware of) - my findings are new and it's quite a change around from 'accepted arboricultural theory' (especially Claus Mattheck's previous theories on bark-inclusions), but, fortunately, if you have done a scientific piece of work and you keep finding natural bracing to be relevant in assessing tree form, one can easily live with a bit of skepticism.

I have really changed my arboricultural practice since I discovered the effects of natural bracing. In essence, I'm just an arboriculturist, looking to share my new knowledge with others - what others do with that new knowledge is their business. From my point of view, if some stay skeptical, they still have to take into account natural braces as and when they find them - as natural braces are physical, tangible and self-evident parts of the crown architecture of trees, so you cannot go around denying their existence without looking pretty daft. I am certain that natural bracing will become a standard aspect of arboricultural training - and should probably be shared into horticulture too (as it affects many woody plants - even roses (Rosa spp.)).

This short case study helps identify the high relevance of natural bracing when you look at the causes of a bark-inclusion failure: I have now accumulated a few examples of failures like this case study; I have chosen this one to share, as it is pretty good in terms of 'visuals'.

Figure 1: Semi-mature sallow (Salix x reichardtii) which has suffered a bark-inclusion failure this winter (2017-18), splitting one of its main stems in two directions. This example is on Myerscough College's Preston campus, Lancashire.

Figure 2: The splayed split branches show a typical 'V'-shaped wedge of included bark at the apex of the junction. The white arrow identifies that a smaller lateral branch was rubbing against something else in the structure of this sallow, before the failure of this bark-included junction occurred.

Figure 3: The underside of a lateral branch on one of the failed limbs arising from the bark-inclusion, shows that it was pressing against the lateral branch shown in Figure 2. The rubbing marks match up perfectly, as does the height of the two branches, if you re-erected the fallen stem.

This match-up between the rubbing part of two lateral branches (Fig.s 2 and 3) shows that the junction was previously "naturally braced" - as I have decided to call this effect. I love this type of detective work that is involved in analysing a tree failure: I have now done this for five bark-inclusion failures, four of which had very clear evidence that natural bracing was involved - in the fifth one, the possibility of natural bracing being involved could not be dismissed, but evidence was only circumstantial.

Here, in this case, the process is pretty clear: a lateral branch from a third stem (Fig. 2, arrowed) braced this junction at an early stage of the development of this sallow - the junction did not form normally as a consequence (Fig.s 1 & 2) and it became a bark-included junction due to a lack of mechanical stimulus at its apex. This junction consisted of two upright limbs that grew rapidly, whereas the lateral branch remained small, and it only had a rubbing connection to this junction. As this sallow grew larger, and the two upright limbs that were attached to the weak junction grew longer, heavier and more exposed to the wind, this resulted in this failure in a winter storm. This happened because the natural brace was not developing at the same rate, and its contact with the junction was only superficial (not fusion or entwining).

Figure 4: In the remaining standing stem of this sallow, you can see it is a type of 'tree' (well, is it a multi-stemmed tree, or is it a shrub? - you decide) that readily forms natural braces, as this last stem also has a bark inclusion and a very obvious (for now) natural brace in position. But how long will this minor lateral branch survive in this fast-growing hybrid? - not that long, before it is shaded out, it dies, decays, and the bark-included junction is thus opened up to movement. If an arborist turned up to inspect the tree at this later stage, all they would see was the bark-included junction, not the natural brace that caused its formation. This pattern of behaviour is one I get to see again and again in many species of tree and shrub.

Apart from loving the "detecting" part of analysing such tree failures, looking for natural bracing in trees has resulted in a radical change in my pruning and tree assessment practices - and how I teach about trees. I like that it is strongly evidence-based (not conjecture) and that, as arboriculturists, we can actually do something about this effect - especially, formatively prune young trees to prevent natural braces from forming, and so preventing the vast majority of bark inclusions forming.

Please note (especially the skeptics) that one does not always get evidence of natural bracing when finding a failed bark-included junction - and that is for a number of reasons. The main three reasons are:

• From my research, it would seem that a small proportion of bark-included junctions cannot be explained by natural bracing, so there must be at least one (if not several) other minor causes of weak junctions. However, in three large cohorts of trees that we have studied thus far, the unexplained bark-inclusions were few (less than 7% of all bark-included junctions).
• Natural braces can be quite superficial and leave no mark on the two main branches or stems that fail. In particular, it is quite common that the natural brace takes the form of two lateral twigs/small branches touching each other: if those lateral growths that form the brace are shaded out, one only has the knots on the two main limbs above the bark inclusion as any sort of evidence that there was a natural brace present. In such cases, one cannot dismiss the potential for natural bracing to have been involved in forming the bark-inclusion, but there is no easy means of evidencing whether it was natural braced or not (unless you have a time machine and travel back to an earlier stage of the tree's development). All one can say, based on my research thus far, is that natural bracing is the primary cause of bark-inclusions in a wide range of species, and it is more likely than not, when encountering a bark inclusion, that it was previously braced in one way or another. I have even encountered young nursery stock with bark-inclusions in, but no obvious natural brace present - but, then, it turned out this stock was "braced" in production at the nursery by their own support systems or by their branches touching other trees in their row.
• Natural braces are often pruned out by arborists (as they see them as 'faults' - e.g rubbing branches), and then junction failure occurs two or more years later. The only evidence that you have in such cases are the cuts that the arborist made to the stem(s) - often the evidence that what was cut out was a natural brace is not there to be seen (often for the same reason as given above - that one or two lateral branches were cut off, which formed the natural brace).

There is, more often than not, a tangible, rational explanation that involves bracing (artificial or natural), basically, for the vast majority of bark inclusions formed in trees - and that's very useful knowledge for arboriculturists - it solves the otherwise 'mysterious' appearance of bark included junctions in trees.

I hope sharing this case study is useful for those that have read it through. As well as my short book on 'Assessing Tree Forks', available from the Arboricultural Association at ￡15, my two main papers about natural bracing are being published in the next edition of the Arboricultural Journal (Spring edition, 2018) - so that's another way to share this new finding with arborists and other arboricultural practitioners - as well as the many talks I have given and will be giving - and as well as training all those taking qualifications with us at Myerscough College now - and in the foreseeable future.

With best wishes,

Duncan.